Methods and apparatus for blood speckle detection in an intravascular ultrasound imaging system

ABSTRACT

Methods and apparatus for blood speckle detection for enhanced intravascular ultrasound imaging. The present invention utilizes the fact that the energy scattering strength from blood exhibits a high frequency dependency, while the scattering strength from tissue lacks a strong frequency dependency. In specific embodiments, the present invention may provide a particularly simple and useful solution for addressing the problem of blood speckle in intravascular ultrasound imaging, especially in situations where the blood may have a scattering strength similar to that of tissue and/or where the blood is moving slowly or not at all.

BACKGROUND OF THE INVENTION

The present invention relates to high resolution intravascular imagingand more particularly to intravascular ultrasound imaging and techniquesfor enhancing image quality.

In intraluminal or intravascular ultrasound (also referred to as "IVUS")imaging, the production of high resolution images of vessel wallstructures requires imaging at high ultrasound frequencies. In sometypes of intraluminal systems, an ultrasonic unidirectionalexciter/detector within a catheter probe positioned within a bloodvessel is used to acquire signal data from echoes of the emittedultrasonic energy off the interior of the blood vessel. Specifically,vectors are created by directing focused ultrasonic pressure wavesradially from a transducer in a catheter and collecting echoes at thesame transducer from the target area. A plurality of radial vectors fromthe rotated transducer comprises an image frame. A signal processorperforms image processing (e.g., stabilization of a moving image,temporal filtering for blood speckle, and other image enhancementtechniques) on the acquired data in order to provide a display of thecorrected and filtered intravascular image on a raster-scan displaymonitor.

It is desirable to provide imaging over a broad range of frequencies(e.g., 5 Megahertz (MHz) to 50 MHz), especially higher ultrasonicfrequencies in some applications. However, the backscatter from bloodcells in such an image is a significant problem in high frequencyintraluminal ultrasound imaging, since the scattering of ultrasound fromblood cells is proportional to the fourth power of the frequency suchthat the higher the ultrasound frequency the more pronounced is thebackscatter from blood. As a result, echoes from blood molecules degradethe lumen-to-vessel wall contrast, which is undesirable since there is aneed to define the blood/tissue boundary in order to ascertain thedegree of narrowing of the vessel and to determine the spatial extent ofthe plaque. Therefore, echoes in the ultrasound image due to backscatterfrom blood (the irregular pattern of backscatter from blood is referredto as "blood speckle") must be detected in order to provide an enhancedimage display. Once detected, the blood speckle may be removed orsuppressed to a level at which wall structures can be distinguished fromblood, distinguished by providing a different display color for theblood, and/or used to better delineate the blood/tissue interface.

Various techniques have been used in intravascular ultrasound imagingfor detecting blood speckle in the image. These techniques are notalways effective in distinguishing between blood and tissue, becausethey are based on key assumptions which are not always true. Sometechniques rely on the assumption that the energy scattering strengthfrom blood is low in comparison to the scattering strength from tissue,in order to distinguish between blood and tissue. Other techniques relyon the assumption that the blood moves much faster compared to thetissue and thus has a different Doppler signal than the tissue. Inreality, however, such assumptions may be violated. In particular, theenergy scattering from blood can sometimes be equally as bright as thescattering from tissue, and/or blood may sometimes move with very lowvelocity or not be moving at all. Although generally effective, thesetechniques may not be so effective in situations when these assumptionsare not valid.

From the above, it can be seen that alternative or supplementary methodsand apparatus are needed for detecting blood speckle to allow a displayof intraluminal ultrasound images to be free of or to distinctlyidentify blood-induced echoes.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus which detect bloodspeckle in an improved manner. The present invention utilizes the factthat the energy scattering strength from blood exhibits a high frequencydependency, while the scattering strength from tissue lacks a strongfrequency dependency. In specific embodiments, the present invention mayprovide a particularly simple and useful solution for addressing theproblem of blood speckle in intravascular ultrasound imaging, especiallyin situations where the blood may have a scattering strength similar tothat of tissue and/or where the blood is moving slowly or not at all.

According to a specific embodiment, the present invention provides amethod of detecting blood speckle in an intravascular ultrasound bloodvessel image. The method includes the steps of illuminating anintravascular target with ultrasonic RF energy to generate ultrasonicechoes from the intravascular target, and transforming the ultrasonicechoes from the intravascular target into a received RF signal. Themethod also includes performing spectral analysis on at least a portionof the received RF signal to provide intensity information on thespectrum of the received RF signal. The information includes a firstintensity strength at a high frequency within the spectrum and a secondintensity strength at a low frequency within the spectrum. The methodfurther includes comparing the first intensity strength and the secondintensity strength, and determining that the intravascular target istissue if the first intensity strength and the second intensity strengthare approximately equal and that the intravascular target is blood ifthe first intensity strength is greater than the second intensitystrength. This determining step takes into account strengthsensitivities at the high and low frequencies. Some specific embodimentsmay perform spectral analysis either by complete Fourier analysis or byfiltering for the high and low frequencies.

According to another specific embodiment, the present invention providesa method of detecting blood speckle in an intravascular ultrasound bloodvessel image that includes the steps of illuminating an intravasculartarget with ultrasonic RF energy at a first frequency to generateultrasonic echoes from said intravascular target to form a first imageframe, and illuminating the intravascular target with ultrasonic RFenergy at a second frequency to generate ultrasonic echoes from theintravascular target to form a second image frame. The first and secondimage frames are successive in time and one of the first and secondfrequencies is a low frequency with the other one being a highfrequency. The method also includes step of subtracting the first andsecond image frames to obtain a subtracted image frame and the step ofdetermining that portions of the subtracted image frame that aresubstantially cancelled-out are tissue and that portions of thesubtracted image frame that are not cancelled-out are blood. Thedetermining step takes into account strength sensitivities at the highand low frequencies.

According to yet another specific embodiment, the present inventionprovides an apparatus for an ultrasonic blood vessel imaging system. Theapparatus includes a transducer having a frequency bandwidth includingknown and sufficiently high strength sensitivities at a first frequencyand a second frequency. The transducer obtains echoes from anintravascular target using ultrasounds transmitted at the first andsecond frequencies to form an intravascular image. The first and secondfrequencies are between a -3 dB low frequency and a -3 dB high frequencyof the transducer. The apparatus also includes a signal processingdevice and a computer-readable medium. The signal processing device iscapable of being coupled to the transducer and to a display fordisplaying the intravascular image. Coupled to be read by the signalprocessing device, the computer-readable medium stores acomputer-readable program for comparing a first intensity strength forechoes from ultrasound at the first frequency with a second intensitystrength for echoes from ultrasound at the second frequency to detectblood speckle in the intravascular image.

These and other embodiments of the present invention, as well as itsadvantages and features, are described in more detail in conjunctionwith the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of an intravascular ultrasonic imaging systemin accordance with specific embodiments of the invention;

FIG. 1B is a simplified diagram of the power sensitivity of a transduceras a function of frequency, in accordance with specific embodiments ofthe invention;

FIG. 2 is a simplified flow diagram illustrating a specific embodimentthat analyzes the entire spectrum to distinguish between blood andtissue;

FIG. 3 is a simplified flow diagram illustrating another specificembodiment that performs spectral analysis at only two discretefrequencies to distinguish between blood and tissue; and

FIG. 4 is a simplified flow diagram illustrating a further specificembodiment that utilizes a high frequency and a low frequency to obtaintwo successive image frames used to distinguish between blood andtissue.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention provides for the accurate discrimination betweenblood and tissue for enhanced image processing in intravascularultrasound imaging systems. The present invention may use spectralanalysis to distinguish blood from tissue, according to specificembodiments. In particular, the present invention utilizes the fact thatthe energy scattering strength from blood (i.e., blood cells, which areon the order of about 2 micrometer (μm) thick and about 7 μm diameter,are particles much smaller than the wavelength of the ultrasound energy)exhibits a high frequency dependency, while the scattering strength fromtissue lacks a strong frequency dependency. That is, for scattering dueto blood, the scattering intensity at higher frequencies is muchstronger than the energy scattering at lower frequencies. Since thespectrum provides information on any frequency dependency that mayexist, examining the spectrum can provide information about the size ofthe reflectors to indicate whether the reflectors are blood or tissue.

The present invention provides image processing methods which may beused in conjunction with the intravascular ultrasonic imaging systemshown in FIG. 1A. Referring to FIG. 1A, there is shown a block diagramof a type of intravascular ultrasonic imaging system 10 that may be usedfor intravascular image display in accordance with the invention. Asseen in FIG. 1A, a specialized signal processing device 10 is used withan ultrasonic imaging system 12 including a catheter probe 13 whereinultrasonic beams 14 are emitted by an ultrasonic transmitter or exciter16. The ultrasonic signals 14 of, for example, 5 MHz to 50 MHz, aredirected to an intravascular target to cause reflections in the form ofultrasonic echo signals 18 from the intravascular structures, includingblood. Radial spokes or vectors 18 of information are collected from atarget 20 (the interior walls of a blood vessel) based on ultrasonicreflections at a transducer 22. Specifically, information is gathered byprojecting narrow ultrasonic sampling beams 14 from exciter 16 as it isrotated (by an angle θ) within catheter 13 within blood vessel 20. Thereflections scale in amplitude over a range and are recorded bytransducer 22 as amplitude as a function of unit distance (r) along theradius of each vector. A total of, for example, 256 spokes radiallydirected from the catheter 13 is sufficient to obtain data for an imageframe to process the information according to a specific embodiment ofthe present invention. This image data acquisition may provide eitheranalog or digital information, depending on the specific systemutilized. The data acquired is converted into pixels representing pointsin a scanned (swept or rotated) two-dimensional image are assigned avalue on, for example, a gray scale between black and white. Of course,colors may be assigned in other embodiments. The image is representativeof a cross-sectional "slice" of the structure of blood vessel 20 andincludes wall structures (blood-wall interface) 26 and lumens of blood(blood region) 24, as seen in FIG. 1A. More specifically, after theintravascular ultrasonic imaging system acquires the image data, signalprocessor 10 performs signal processing of the acquired image data byscan-converting the image data into x-y rasterized image data forstoring into display memory 32 and then stabilizing the rasterized imagedata on a frame-by-frame basis to provide the raster image for viewingon a display device 30 coupled to signal processor 10. Signal processor10 also includes a program memory 38 which may be used to store thecomputer-readable program(s) for implementing specific embodiment(s) ofthe present invention, as discussed further below. Alternatively, thecomputer-readable program(s) for implementing specific embodiments ofthe present invention may be stored on a memory coupled to signalprocessor 10. For example, the memory may be a read-only memory, fixeddisk drive, or removable disk drive. The present invention can be usedto distinguish or suppress/remove blood speckle in the displayed image.

According to a specific embodiment of the present invention, the radiofrequency (RF) of the echoes would be acquired and then analyzed in thefrequency domain using Fourier analysis to compute the spectrum, as iswell known in the art. FIG. 2 is a simplified flow diagram illustratinga specific embodiment that analyzes the entire spectrum. It is notedthat the associated electronics of the apparatus in order to acquire theRF echoes would have to deal with a higher frequency as well as have ahigher dynamic range compared to apparatus used with an approach whichacquires the log-compressed envelope of the reflected echoes. Accordingto this specific embodiment, the transducer transmits RF along itsentire bandwidth (indicated as step 51) and receives RF echo signals(step 55). Computed using Fourier analysis (step 59), the power spectrumof the RF echo signals characterizes the nature of the reflectors toprovide information for better distinguishing between blood and tissue.In this specific embodiment, after the RF is acquired and spectralanalysis is performed, the strength of the received RF signal at the twofrequency bins are compared. In particular, the strength of the spectrumin two frequency bins (a higher frequency bin and a lower frequency bin)where the transducer has known sensitivities are examined. Specifically,this embodiment requires the use of a transducer with a wide bandwidthwhich includes a lower frequency bin and a higher frequency bin havingsubstantially well known and sufficiently high sensitivities. As shownin FIG. 1B, which is a simplified diagram of the power sensitivity of atransducer (the transducer has a center frequency f₀ at which thetransducer has a peak power, P_(PEAK)) as a function of frequency, boththe higher and lower frequency bins are preferably selected to fallwithin the range between the -3 dB low frequency f_(-3dB) LOW (thefrequency below f₀ at which power is half of P_(PEAK)) and the -3 dBhigh frequency f_(-3dB) HIGH (the frequency above f₀ at which power ishalf of P_(PEAK)). In a preferred embodiment, the higher and lowerfrequency bins are both selected to fall within the range between the -3dB low frequency f_(-3dB) LOW and f₀. However, in alternativeembodiments, the higher and lower frequency bins may be selected to fallwithin the range between f₀ and the -3 dB high frequency f_(-3dB) HIGH.In another alternative embodiment, for example, the lower frequency binmay be selected to fall within the range between the transducer's centerfrequency f₀ (the frequency at which the transducer has a peak power,P_(PEAK)) and the -3 dB low frequency f_(-3dB) LOW (the frequency belowf₀ at which power is half of P_(PEAK)), and the higher frequency bin maybe selected to fall within the range between the transducer's centerfrequency f₀ and the -3 dB high frequency f_(-3dB) HIGH (the frequencyabove f₀ at which power is half of P_(PEAK)). The two frequency binsshould also be selected to be as separate as possible from each other(so that the bandwidths of each frequency bin do not overlap or are nottoo close to each other) without falling out of the range of thetransducer's frequencies with known and sufficiently high sensitivities.For example, for frequency bins selected close to the center frequency,more narrowband frequency bins should be used. For frequency binsselected further away from the center frequency, wider band frequencybins may be used as long as the bins remain within the -3 dBfrequencies. A comparison of the strength of the spectrum at those twofrequency bins (taking into account the particular strengthsensitivities at each bin) determines whether the echoes were reflectedfrom tissue or from blood. If the strength of the spectrum at those twofrequencies is approximately equal (taking into account the knownsensitivities of the transducer at each frequency bin) as indicated instep 63, then the echoes were reflected from tissue and the particularpixel is determined to be tissue (indicated by step 65). If the higherfrequency bin has a greater strength than the lower frequency bin (alsotaking into account the known sensitivities of the transducer at eachfrequency bin) as indicated in step 67, then the reflected echoes camefrom blood and the particular pixel is determined to be blood (step 69).This embodiment performs an analysis of the entire spectrum with thesteps shown in FIG. 2 being performed for each radial spoke and thecomparison of strength for the high and low frequency bins beingperformed for each sampling point in the radial spoke. In an exemplaryimplementation of this specific embodiment, the transducer has a centerfrequency of about 40 MHz with about a total 20 MHz bandwidth, and theanalysis and examination of the entire spectrum would becomputation-intensive, as a complete Fourier analysis is involved. Thisspecific embodiment may be desirable in some applications, since theinformation obtained (such as or including the spectral analysis for theentire spectrum) may be useful for other purpose in addition todetecting blood speckle.

In another specific embodiment, the spectral analysis may be performedat two predetermined discrete frequencies for the transducer in thecatheter. FIG. 3 is a simplified flow diagram illustrating the specificembodiment that performs spectral analysis at only two discretefrequencies. It is noted that this embodiment also requires the use of atransducer with a wide bandwidth which includes a lower frequency f₁ anda higher frequency f₂ at which the transducer has substantially wellknown and sufficiently high sensitivities, as discussed above for FIG.1B. The two frequencies are selected to have known sensitivities for theparticular transducer in the catheter and to fall within the preferredfrequency range (between the -3 dB high and low frequencies, asdiscussed above). The following discussion also assumes that thestrength comparison at the two discrete frequencies takes into accountthe known sensitivities of the transducer at the respective frequencies,in a similar manner as discussed for the embodiments of FIG. 2.According to this specific embodiment, the transducer transmits RF alongits entire bandwidth (indicated as step 91) and receives RF echo signals(step 95). In the present embodiment, spectral analysis is performedwithout having to perform Fourier analysis of the RF signal to providethe entire spectrum. Instead, the spectral analysis is performed (step97) at the two discrete frequencies, lower frequency f₁ and a higherfrequency f₂, by bandpass filtering. In one specific embodiment, thebandpass filtering is performed with a respective set of coefficientsthat are available through a look-up table (LUT), which may be includedin (e.g., LUT 40 shown in dotted line in FIG. 1A) or coupled to (e.g.,LUT 42 shown in dotted line in FIG. 1A) the signal processor 10 of FIG.1A. In another specific embodiment, the bandpass filtering may beperformed using hardware bandpass filters in imaging system 12 at eachof the lower and higher frequencies. These embodiments thus avoid theneed to do a complete Fourier analysis of the RF echo signal. Acomparison (step 99) of the strength at those two discrete frequenciesdetermines whether the echoes were reflected from tissue or from bloodin the present embodiment, in a similar manner as the embodimentdescribed in FIG. 2. That is, if the strength of the spectrum at thosetwo frequencies is approximately equal (indicated in step 101), then theechoes were reflected from tissue and the particular pixel is determinedto be tissue (indicated by step 103). If the higher frequency f₂ has agreater strength than the lower frequency f₁ (indicated in step 105),then the reflected echoes came from blood and the particular pixel isdetermined to be blood (step 107). This embodiment performs a spectralanalysis and intensity-based comparison of the received RF signal at thetwo frequencies with the steps shown in FIG. 3 being performed for eachradial spoke and the comparison of strength at the high and lowfrequencies being performed for each sampling point in the radial spoke.In an exemplary implementation, the transducer has a center frequency ofabout 40 MHz with about a total 20 MHz bandwidth and the apparatus mayhave lower processing and memory requirements, since the presentembodiment is less computation-intensive by avoiding a complete Fourieranalysis of the entire spectrum of the RF signal.

It should be noted that although exemplary implementations discussed forthe previous two specific embodiments may use wideband transducers witha center frequency of about 40 MHz with about 20 MHz bandwidth, othertypes of transducers may be used in other exemplary implementations. Asexamples, wideband transducers having a center frequency/bandwidth rangeas follows may be used: about 9 MHz with about 3.6-5.4 MHz bandwidth;about 12 MHz with about 4.8-7.2 MHz bandwidth; or 30 MHz with about12-18 MHz bandwidth. Other wideband transducers with even higher centerfrequencies, such as a transducer of about 100 MHz with about 40-50 MHzbandwidth, may be used, as long as the higher frequency or frequency binused for the above two specific embodiments have correspondingwavelengths that are greater than the typical diameter (about 7 μm) ofblood cells. It is noted that the transducer mounted in a catheter usedin IVUS imaging systems currently provide information to the imageprocessor through its catheter ID. Such information includes the centerfrequency of the particular transducer, and additional information thatmay be provided can include the -3 dB high frequency and the -3 dB lowfrequency, and/or the entire sensitivity power spectrum of theparticular transducer, which may be used in accordance with the presentinvention.

In still another specific embodiment, the need to perform spectralanalysis and the need for a wide bandwidth transducer are eliminated, asexplained further below. In the present specific embodiment, a widebandwidth transducer may be used and the high frequency and the lowfrequency channels used with the wide bandwidth transducer may be widebandwidth (i.e., shorter pulses) which are sufficiently separated fromeach other but within the range of -3 dB high and low frequencies, toaccount for known and sufficiently high sensitivities of the transducer.However, the present embodiment also allows for the use of a narrowbandwidth transducer where the high frequency and the low frequencychannels used with the narrow bandwidth transducer have narrowerbandwidths (i.e., longer pulses) which are sufficiently separated fromeach other but outside the range of -3 dB high and low frequencies, toaccount for known sensitivities of the transducer. FIG. 4 is asimplified flow diagram illustrating this specific embodiment thatutilizes a high frequency and a low frequency to obtain two successiveimage frames used to distinguish between blood and tissue. In thisspecific embodiment (described for a narrow bandwidth transducer forsimplicity), the transducer can transmit two narrowband tones at the twofrequencies (high and low), where the transducer has known andsufficiently high sensitivities for the two frequency tones. Asindicated by step 111, the transducer transmits a narrowband lowfrequency tones at f₁ to obtain a first image frame. Then, thetransducer transmits a narrowband high frequency tones at f₂ to obtain asecond image frame (step 113). As mentioned earlier, a plurality ofradial vectors from the rotated transducer comprises an image frame. Ofcourse, in other embodiments, the first image frame may be obtained byusing a high frequency tone or channel and the second image frame may beobtained by using a low frequency tone or channel, as long as thesuccessive image frames are obtained by a high frequency tone and a lowfrequency tone. The two successive images are subtracted in step 115. Asindicated by step 117, the tissue portion would be largely cancelled andthe blood portion would not, due to the fact that the reflected echoes'strengths between the two tones' frequencies would be similar for tissueand different for blood. The subtracted image information may then beused, for example, as a mask for removing blood speckle in the displayedimage. Of course, this embodiment would incur the time to obtain twoimage frames for determining the blood's spatial distribution. In thisembodiment, the bandwidth of each channel may be in the kilohertz (kHz)range with the channels separated from each other as much as possiblebut having both channels within the range of known sensitivities of thetransducer, as discussed above for FIG. 1B. It should be recognized thatthe above discussion for this embodiment also assumes that the knownsensitivities of the transducer at the high and low frequency tones aretaken into account, in a similar manner as discussed for the embodimentsof FIG. 2 with respect to the known sensitivities.

Because the present invention utilizes RF digitization, betterdigitization is required (i.e., more samples are required) so that notonly are the signals' envelope detected but also individual signals needto be detected so that the analysis can be narrowed down. For somespecific applications where transducers with f₀ of lower frequenciessuch as 10 MHz are required, direct sampling digitization may be used;whereas, known techniques for higher RF digitization may be utilized forspecific applications where transducers with f₀ of higher frequenciessuch as 40 MHz are required.

The present invention may be used as the sole means for blood speckledetection, or as an adjunct for conventional intensity-based andmotion-based analysis for blood speckle detection used to delineate thelumen and vessel wall boundary. Accordingly, the present inventionprovides an improved capability for detecting blood for applicationssuch as assigning a distinct color to the detected blood in thedisplayed image, or suppressing or removing completely the detectedblood from the displayed image. While the invention has beenparticularly shown and described with reference to preferred embodimentsthereof, it will be understood by those skilled in the art that theforegoing and other changes in the form and details may be made thereinwithout departing from the spirit or scope of the invention. It istherefore not intended that this invention be limited, except asindicated by the appended claims.

What is claimed is:
 1. A method of distinguishing tissue from blood inan intravascular ultrasound blood vessel image, said method comprisingthe steps of:illuminating an intravascular target with ultrasonic RFenergy to generate ultrasonic echoes from said intravascular target;transforming the ultrasonic echoes from the intravascular target into areceived RF signal; performing spectral analysis on at least a portionof said received RF signal to provide intensity information on thespectrum of said received RF signal, said information including a firstintensity strength at a high frequency within said spectrum and a secondintensity strength at a low frequency within said spectrum; comparingsaid first intensity strength and said second intensity strength;determining that said intravascular target is tissue if said firstintensity strength and said second intensity strength are approximatelyequal and that said intravascular target is blood if said firstintensity strength is greater than said second intensity strength,wherein said determining step takes into account tissue and bloodbackscatter strength sensitivities at said high and low frequencies. 2.The method of claim 1 wherein said step of performing spectral analysisis achieved by performing a complete Fourier analysis on said receivedRF signal such that said information provided is for the entirespectrum.
 3. The method of claim 2 wherein a transducer used for saidillumination step has known and sufficiently high detectionsensitivities at both said high and low frequencies and wherein said lowand high frequencies are selected to be between a low -3 dB frequencyand a high -3 dB frequency of said transducer.
 4. The method of claim 1further comprising the step of:selecting a narrowband high frequencychannel containing said high frequency and a narrowband low frequencychannel containing said low frequency.
 5. The method of claim 4 whereina transducer used for said illumination step has known and sufficientlyhigh detection sensitivities at both said narrowband high and lowfrequency channels, and wherein said narrowband high frequency channeland said narrowband low frequency channel are selected to be between alow -3 dB frequency and a high -3 dB frequency of said transducer. 6.The method of claim 4 wherein a transducer used for said illuminationstep has known and sufficiently high detection sensitivities at bothsaid narrowband high and low frequency channels, and wherein saidnarrowband high frequency channel and said narrowband low frequencychannel are selected to be between a center frequency and a high -3 dBfrequency of said transducer.
 7. The method of claim 1 wherein said stepof performing spectral analysis is achieved by filtering at said highfrequency and at said low frequency.
 8. The method of claim 7 whereinsaid filtering is performed by using respectively appropriate sets offilter coefficients stored in a memory such that said informationprovided is for said high frequency and for said low frequency.
 9. Themethod of claim 8 wherein said filtering is performed using a look-uptable as said memory.
 10. The method of claim 7 wherein said filteringis performed by using hardware bandpass filters for said high frequencyand for said low frequency.
 11. The method of claim 1 wherein saidilluminating step is performed with a transducer having a centerfrequency and a bandwidth of about 40-60% of said center frequency,wherein said high frequency is a frequency having a correspondingwavelength that is greater than the typical diameter of blood cells. 12.The method of claim 1 further comprising the steps of:assigning saidintravascular target a selected first shade if said first intensitystrength and said second intensity strength are determined to beapproximately equal and a selected second shade if said first intensitystrength is determined to be greater than said second intensitystrength; and providing said intravascular ultrasound blood vessel imagewith tissue having said selected first shade and blood having saidselected second shade on a display.
 13. The method of claim 1 whereinsaid selected second shade is selected such that said blood issuppressed or removed from said intravascular ultrasound blood vesselimage on said display.
 14. A method of distinguishing tissue from bloodin an intravascular ultrasound blood vessel image, said methodcomprising the steps of:illuminating an intravascular target withultrasonic RF energy at a first frequency to generate ultrasonic echoesfrom said intravascular target to form a first image frame; illuminatingsaid intravascular target with ultrasonic RF energy at a secondfrequency to generate ultrasonic echoes from said intravascular targetto form a second image frame, wherein said first and second image framesare successive in time and wherein one of said first and secondfrequencies is a low frequency and another one of said first and secondfrequencies is a high frequency; subtracting said first and second imageframes to obtain a subtracted image frame; determining that portions ofsaid subtracted image frame that are substantially cancelled-out aretissue and that portions of said subtracted image frame that are notcancelled-out are blood, wherein said determining step takes intoaccount tissue and blood backscatter strength sensitivities at said highand low frequencies.
 15. The method of claim 1 wherein a transducer usedfor said illumination steps has known and sufficiently high detectionsensitivities at both said high and low frequencies, and wherein saidhigh frequency is selected to be between a center frequency and a high-3 dB frequency of said transducer, and said low frequency is selectedto be between the center frequency and a low -3 dB frequency of saidtransducer.
 16. The method of claim 15 further comprising the stepsof:assigning said portions of said subtracted image frame that are notcancelled-out a selected shade; and providing said intravascularultrasound blood vessel image with blood having said selected shade on adisplay, said selected shade being different from other shades fornon-blood in said display.
 17. The method of claim 15 further comprisingthe steps of:providing said intravascular ultrasound blood vessel imageon said display such that said portions of said subtracted image framethat are not cancelled-out are suppressed or removed from saidintravascular ultrasound blood vessel image.
 18. Apparatus for anultrasonic blood vessel imaging system comprising:a transducer having afrequency bandwidth including known and sufficiently high strengthsensitivities at a first frequency and a second frequency, saidtransducer obtaining echoes from an intravascular target usingultrasounds transmitted at said first and said second frequencies toform an intravascular image, wherein said first and second frequenciesare between a -3 dB low frequency and a -3 dB high frequency of saidtransducer; a signal processing device capable of being coupled to saidtransducer and to a display for displaying said intravascular image; acomputer-readable medium storing a computer-readable program, saidcomputer-readable medium coupled to be read by said signal processingdevice, said computer-readable program for comparing a first intensitystrength for echoes from ultrasound at said first frequency with asecond intensity strength for echoes from ultrasound at said secondfrequency to detect blood speckle in said intravascular image.
 19. Theapparatus of claim 18 wherein said computer-readable program comparessaid first and second intensity strengths for the same image frame. 20.The apparatus of claim 18 wherein said computer-readable programcompares said first intensity strength with said second intensitystrength by subtracting a first image frame obtained from echoes fromultrasound at said first frequency with a second image frame obtainedfrom echoes from ultrasound at said second frequency to provide asubtracted image frame, said second image frame and said first imageframe are successive image frames, said subtracted image frame includingportions of said subtracted image frame that are not cancelled-out, saidportions being deleted from or distinctly shaded in said intravascularimage on said display.